Commercial aircraft often comprise air-conditioning systems that are based on an air cycle process that provides sufficient cooling power, ventilation air flow, and pressurization. Such an air-conditioning system typically comprises a compact air-conditioning unit, which is also known as an “air conditioning pack”, is usually arranged, for example, in a wing root or belly fairing of the aircraft, and is supplied with compressed process air. The process air may be generated by a compressor or may be bled from a compressor stage of an engine or an auxiliary power unit (APU) of the aircraft. During flight operation of the aircraft, engine bleed air is used, usually during ground operation of the aircraft, primarily compressed process air from an auxiliary power unit of the aircraft or from a ground unit is used.
In the air-conditioning unit, the process air flows through at least one heat exchanger as well as through various compression and expansion units and is cooled and expanded. Cooled process air exiting the air-conditioning unit is finally supplied to a mixing chamber where it is mixed with recirculation air recirculated from an aircraft region to be air-conditioned. For example, DE 10 2008 053 320 A1 shows such an air-conditioning system.
Often, an aircraft cabin to be conditioned is separated into a number of different temperature (control) zones. The number of temperature zones depends on the size of the aircraft and a desired cabin layout. Every temperature zone usually has a dedicated zone temperature sensor, which measures the temperature in the respective temperature zone. A control unit for controlling the air conditioning system receives temperature signals from all temperature sensors in the aircraft and controls the air-conditioning system such that the temperatures in the temperature zones meet the desired temperatures.
Typically, the temperature zone with the highest cooling demand determines the reference temperature for the air supplied to the cabin from a centralized mixer unit, which is fed by the conditioned process air of the air-conditioning system and the recirculated air. In order to prevent an excessive cooling for all other temperature zones with less cooling demand, individual air supply to these temperature zones will be heated by electrical heaters or through a controlled supply of hot trim air into the individual supply ducts.